The Confederate government increasingly molded its policies in the interest of the planter class. - Eric Foner

Slavery was a byproduct of the quest for profit—economic actors were the architects of the slave plantation systems that came to dominate American society from the Southern United States to the Caribbean and Brazil. It was merchants and planters, not statesmen and jurists, who determined that slave labor was more profitable than free. ... This history constitutes a welcome corrective to the fashionable view that free markets automatically promote social justice.

Essentially what Lincoln said is slavery is a form of theft, the theft of labor, one person stealing another person's labor without that person’s permission.

Now, as then, commerce unrestrained by social control ends up stripping economic relations of all moral content, while the drive toward fulfilling market demands at the lowest possible price creates widespread indifference to the conditions under which marketable goods are produced. Today’s Chinatown sweatshops and Third World child labor factories are the functional equivalents of colonial slavery in that the demands of the consumer and the profit drive of the entrepreneur overwhelm the rights of those whose labor actually produces the salable commodity.